This invention relates to hydrophobic polymer housings used on electrical equipment, such as surge arresters.
Electrical transmission and distribution equipment is subject to voltages within a fairly narrow range under normal operating conditions. However, system disturbances, such as lightning strikes and switching surges, may produce momentary or extended voltage levels that greatly exceed the levels experienced by the equipment under normal operating conditions. These voltage variations often are referred to as over-voltage conditions.
If not protected from over-voltage conditions, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed by over-voltage conditions and associated current surges. Accordingly, it is routine practice for system designers to use surge arresters to protect system components from dangerous over-voltage conditions.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert over-voltage-induced current surges safely around the equipment, and to thereby protect the equipment and its internal circuitry from damage. When exposed to an over-voltage condition, the surge arrester operates in a low impedance mode that provides a current path to electrical ground having a relatively low impedance. The surge arrester otherwise operates in a high impedance mode that provides a current path to ground having a relatively high impedance. The impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester when the surge arrester is operating in the low-impedance mode, and is otherwise substantially higher than the impedance of the protected equipment.
When the over-voltage condition has passed, the surge arrester returns to operation in the high impedance mode. This high impedance mode prevents normal current at the system frequency from flowing through the surge arrester to ground.
Conventional surge arresters typically include an elongated outer enclosure or sheath made of an electrically insulating material, such as porcelain or a polymeric material, a pair of electrical terminals at opposite ends of the enclosure for connecting the arrester between a line-potential conductor and electrical ground, and an array of other electrical components that form a series electrical path between the terminals. These components typically include a stack of voltage-dependent, nonlinear resistive elements, referred to as varistors. A varistor is characterized by having a relatively high impedance when exposed to a normal system frequency voltage, and a much lower resistance when exposed to a larger voltage, such as is associated with over-voltage conditions. In addition to varistors, a surge arrester also may include one or more spark gap assemblies electrically connected in series or parallel with one or more of the varistors. Some arresters also include electrically conductive spacer elements coaxially aligned with the varistors and gap assemblies.
In one general aspect, a housing for an electrical apparatus includes a sheath, at least one shed, and a hydrophobic coating. The sheath includes a first electrically insulative material and an outer surface. The shed includes a second electrically insulative material and an outer surface. The hydrophobic coating is applied to the outer surface of at least one of the sheath and the shed. One of the first electrically insulative material and the second electrically insulative material includes an electrically insulative, polymeric material.
Implementations may include one or more of the following features. For example, the sheath may be made from a high temperature vulcanizing (xe2x80x9cHTVxe2x80x9d) silicone, the shed may be made from a room temperature vulcanizing (xe2x80x9cRTVxe2x80x9d) silicone, and the coating may be made from one or more of a liquid silicone (xe2x80x9cLSxe2x80x9d) rubber and a RTV silicone. The coating may form a continuous or a non-continuous surface on the outer surface of the sheath and/or on the outer surface of the shed.
The sheath may be made from a HTV silicone, the shed may be made from a RTV silicone, and the coating may be made from one or more of a LS rubber and a RTV silicone. The coating may form a continuous surface or a non-continuous surface on the outer surface of the sheath.
The sheath may be made from a RTV silicone, the shed may be made from a HTV silicone, and the coating may be made from one or more of a LS rubber and a RTV silicone. The coating may form a continuous surface or a non-continuous surface on the outer surface of the shed.
The first electrically insulative material of the sheath may be one or more of an ethylene-propylene-based material, an ethylene vinyl acetate, a cycloaliphatic resin, and an elastomeric or polymeric insulative material, and the coating may be made from one or more of a LS rubber and a RTV silicone. The coating may form a continuous surface or a non-continuous surface on the outer surface of the sheath and/or the outer surface of the shed.
The second electrically insulative material of the at least one shed may be one or more of an ethylene-propylene-based material, an ethylene vinyl acetate, a cycloaliphatic resin, and an elastomeric or polymeric insulative material, and the coating may be made from one or more of a LS rubber and a RTV silicone. Once again, the coating may form a continuous surface or a non-continuous surface on the outer surface of the sheath and/or the outer surface of the shed.
A kit that includes the coating and a coating applicator also may be provided. The kit is designed to be used after the electrical apparatus has been installed in the field and can be used to apply a coating.
The electrical apparatus may include one or more of a transformer, a capacitor, a switch, a recloser, a circuit breaker, a feed through bushing, a suspension insulator, a dead ends insulator, a post insulator, a pin insulator, and a buss support.
In another general aspect, forming a housing for an electrical apparatus includes providing a sheath, providing at least one shed, and applying a hydrophobic coating. The sheath includes a first electrically insulative material and an outer surface. The shed includes a second electrically insulative material and an outer surface. The hydrophobic coating is applied to the outer surface of at least one of the sheath and the shed.
The coating may be applied to the sheath and sheds of the electrical apparatus after the electrical apparatus has been installed. The coating may be periodically applied as part of a maintenance program.
In another general aspect, a housing for an electrical apparatus includes a polymer sheath, at least one polymer shed, and a hydrophobic RTV silicone coating. The polymer sheath is made from an electrically insulative polymeric material and has an outer surface. The polymer shed is integrally attached to the sheath, is made from the electrically insulative polymeric material, and has an outer surface. The hydrophobic RTV silicone coating is applied to the outer surface of the sheath and to the outer surface of the shed.
Implementations may include one or more of the features described above. In addition, the electrically insulative polymeric material may include one or more of a HTV silicone, a polymer concrete, and an ethylene-propylene rubber.
In another general aspect, maintaining a housing for an electrical apparatus that includes a polymer sheath and at least one polymer shed includes providing a hydrophobic coating and applying the hydrophobic coating to at least one of the polymer sheath and the polymer shed. Implementations may include one or more of the features described above. In addition, the coating can include a pigment that colors the coating a first color that is different from a second color of the polymer sheath and maintaining the housing further includes determining the integrity of the coating by looking for breaks in the color of the coating.
The improved housing provides considerable advantages. For example, the hydrophobic coating on the housing causes water to bead on the surface, which reduces or eliminates conductive paths in which leakage currents and dry band arcing can occur. Such conductive paths can result in degradation of the sheath. Similarly, the hydrophobic coating covers mold lines, which reduce the formation of conductive paths along the mold lines.
A non-continuous hydrophobic coating can be used to break conductive paths by forming intermittent surfaces on which water beads. The hydrophobic coating also provides the considerable advantage of forming a bond to the underlying sheath and sheds that make the coating difficult to scrape off accidentally. The hydrophobic coating may be reapplied as necessary to maintain a hydrophobic surface on the sheath and sheds. Periodically applying or reapplying the hydrophobic coating as part of a maintenance program lengthens the life of the housing.
Other features and advantages will be apparent from the description, the drawings, and the claims.